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Orion Arm
The Orion Arm of the Milky Way Galaxy and the Sagittarius arm, which is the next arm “inward.” Earth, and most of known space, lies near the edge of the Orion arm close to the other sections of the galaxy. M☉ |label8=Sun's distance to galactic center |data8=26,000 ± 1,400 light-years |label9=Sun's galactic rotation period |data9=220 million years (negative rotation) |label10=Spiral pattern rotation period |data10=50 million years | title=Gas dynamics in the Milky Way: second pattern speed and large-scale morphology | year=2003 | author=Bissantz, Nicolai | journal=Monthly Notices of the Royal Astronomical Society | volume=340 | pages=949 }} |label11=Bar pattern rotation period |data11=15 to 18 million years |label12=Speed relative to CMB rest frame |data12=552 km/s |below= See also: Galaxy, List of galaxies }} The Milky Way, or simply the Galaxy, is the galaxy in which the Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies. It is one of billions of galaxies in the observable universe. Its name is a translation of the Latin Via Lactea, in turn translated from the Greek Γαλαξίας (Galaxias), referring to the pale band of light formed by the galactic plane as seen from Earth (see [[Galaxy#Etymology|etymology of galaxy]]). Some sources hold that, strictly speaking, the term Milky Way should refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy should receive the full name Milky Way Galaxy, or alternatively the Galaxy. However, it is unclear how widespread this convention is, and the term Milky Way is routinely used in either context. Appearance from Earth The Milky Way galaxy, as viewed from Earth, itself situated on a spur off one of the spiral arms of the galaxy (see Sun's location and neighborhood), appears as a hazy band of white light in the night sky arching across the entire celestial sphere and originating from stars and other material that lie within the galactic plane. The plane of the Milky Way is inclined by about 60° to the ecliptic (the plane of the Earth's orbit), with the North Galactic Pole situated at right ascension 12h 49m, declination +27.4° (B1950) near beta Comae Berenices. The South Galactic Pole is near alpha Sculptoris. The center of the galaxy is in the direction of Sagittarius, and the Milky Way then "passes" (going westward) through Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion & Gemini, Taurus, Auriga, Perseus, Andromeda, Cassiopeia, Cepheus & Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius. The Milky Way looks brightest in the direction of the constellation of Sagittarius, toward the galactic center. Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution. Panoramas Image:Milkyway pan1.jpg|360-degree photographic panorama of the galaxy. Image:Deathvalleysky nps big.jpg|A panorama of the Milky Way, as seen from Death Valley, 2007. Size .]] The stellar disk of the Milky Way galaxy is approximately in diameter, and is concidered to be, on average, about thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass stars, which is highly uncertain. Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around —twice the previously accepted value. As a guide to the relative physical scale of the Milky Way, if it were reduced to 10m in diameter, the Solar System, including the Oort cloud, would be no more than 0.1mm in width (0.001%). The Galactic Halo extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~ . At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky Way. Recent measurements by the Very Long Baseline Array (VLBA) have revealed that the Milky Way is much heavier than some previously thought. The mass of our home galaxy is now considered to be roughly similar to that of our largest local neighbour, the Andromeda Galaxy. By using the VLBA to measure the apparent shift of far-flung star-forming regions when the Earth is on opposite sides of the Sun, the researchers were able to measure the distance to those regions using fewer assumptions than prior efforts. The newer and more accurate estimate of the galaxy's rotational speed (and in turn the amount of dark matter contained by the galaxy) puts the figure at about 254 km/s, significantly higher than the widely accepted value of 220 km/s. This in turn implies that the Milky Way has a total mass equivalent to around 3 trillion Suns, about 50% more massive than some previously thought. Age meteor streaks across the sky just below the Milky Way in August 2007.]] It is extremely difficult to define the age of the Milky Way but the age of the oldest star in the Galaxy yet discovered, HE 1523-0901, is estimated to be about 13.2 billion years, nearly as old as the Universe itself. | title= Discovery of HE 1523-0901, a Strongly r''-Process-enhanced Metal-poor Star with Detected Uranium | year= 2007 | author= Frebel, Anna | journal= The Astrophysical Journal | volume= 660 | pages= L117}} This estimate is based on research by a team of astronomers in 2004 using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397. | title=The age of the Galactic thin disk from Th/Eu nucleocosmochronology | year=2005 | author=Del Peloso, E. F. | journal=Astronomy and Astrophysics | volume=440 | pages=1153}} From this research, the elapsed time between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster was deduced to be 200 million to 300 million years. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the oldest stars in the Milky Way at 13.6 ± 0.8 billion years. Based upon this emerging science, the Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago. Composition and structure . The primary stars of Sagittarius are indicated in red.]] The Galaxy consists of a bar-shaped core region surrounded by a disk of gas, dust and stars forming four distinct arm structures spiralling outward in a logarithmic spiral shape (see Spiral arms). The mass distribution within the Galaxy closely resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely-wound arms. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1990s rather than an ordinary spiral galaxy. Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005 which showed the Galaxy's central bar to be larger than previously suspected. The Milky Way's mass is thought to be about 5.8 solar masses (M☉) comprising 200 to 400 billion stars. Its integrated absolute visual magnitude has been estimated to be −20.9. Most of the mass of the Galaxy is thought to be dark matter, forming a dark matter halo of an estimated 600–3000 billion M☉ which is spread out relatively uniformly. Galactic center is in the Local Spur.]] The galactic disc, which bulges outward at the galactic center, has a diameter of between 70,000 and 100,000 light-years. The distance from the Sun to the galactic center is now estimated at 26,000 ± 1400 light-years, while older estimates could put the Sun as far as 35,000 light-years from the central bulge. The galactic center harbors a compact object of very large mass as determined by the motion of material around the center. The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is newly confirmed to be a supermassive black hole. For a photo see Chandra X-ray Observatory; Jan. 6, 2003. Most galaxies are believed to have a supermassive black hole at their center. The Galaxy's bar is thought to be about 27,000 light-years long, running through its center at a 44 ± 10 degree angle to the line between the Sun and the center of the Galaxy. It is composed primarily of red stars, believed to be ancient (see red dwarf, red giant). The bar is surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the Galaxy, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy. Spiral arms Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. There are believed to be four major spiral arms which all start near the Galaxy's center. These are named as follows, according to the image at right: Outside of the major spiral arms is the Outer Ring or Monoceros Ring, a ring of stars around the Milky Way proposed by astronomers Brian Yanny and Heidi Jo Newberg, which consists of gas and stars torn from other galaxies billions of years ago. As is typical for many galaxies, the distribution of mass in the Milky Way Galaxy is such that the orbital speed of most stars in the Galaxy does not depend strongly on its distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s. Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate and different orbits are expected to have significantly different velocities associated with them. This difference is one of the major pieces of evidence for the existence of dark matter. Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If one believes that the inner parts of the arms rotate faster than the outer part, then the Galaxy will wind up so much that the spiral structure will be thinned out. But this is not what is observed in spiral galaxies; instead, astronomers propose that the spiral arms form as a result of a matter-density wave emanating from the galactic center. This can be likened to a moving traffic jam on a highway — the cars are all moving, but there is always a region of slow-moving cars. Thus this results in several spiral arms where there are a lot of stars and gas. This model also agrees with enhanced star formation in or near spiral arms; the compressional waves increase the density of molecular hydrogen and protostars form as a result. Observations presented in 2008 by Robert Benjamin of the University of Wisconsin-Whitewater suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum-Centaurus arm. The rest of the arms are minor or adjunct arms. See also Halo The galactic disk is surrounded by a spheroid halo of old stars and globular clusters, of which 90% lie within 100,000 light-years, suggesting a stellar halo diameter of 200,000 light-years. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the galactic center. While the disk contains gas and dust which obscure the view in some wavelengths, the spheroid component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk. Recent discoveries have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disc of the Andromeda Galaxy (M31) extends much further than previously thought, the possibility of the disk of our own Galaxy extending further is apparent, and this is supported by evidence of the newly discovered Outer Arm extension of the Cygnus Arm. With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the galactic disk. On January 9, 2006, Mario Jurić and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky found a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Galaxy. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years away. Sun's location and neighborhood The Sun (and therefore the Earth and Solar System) may be found close to the inner rim of the Galaxy's Orion Arm, in the Local Fluff inside the Local Bubble, and in the Gould Belt, at a distance of 7.62±0.32 kpc (~25,000±1,000 ly) from the Galactic Center. Majaess D. J., Turner D. G., Lane D. J. (2009). [http://adsabs.harvard.edu/abs/2009arXiv0903.4206M ''Characteristics of the Galaxy according to Cepheids], MNRAS The Sun is currently 5–30 parsecs from the central plane of the galactic disc. The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. The Sun, and thus the Solar System, is found in the galactic habitable zone. There are about 208 stars brighter than absolute magnitude 8.5 within 15 parsecs of the Sun, giving a density of 0.0147 such stars per cubic parsec, or 0.000424 per cubic light-year (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs of the Sun, giving a density of 0.122 stars per cubic parsec, or 0.00352 per cubic light-year (from List of nearest stars), illustrating the fact that most stars are less bright than absolute magnitude 8.5. The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations often coincide with mass extinction periods on Earth; presumably the higher density of stars close to the galactic plane leads to more impact events. It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year), so it is thought to have completed 20–25 orbits during the lifetime of the Sun and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Galaxy is approximately 220 km/s. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit). Environment created from more than 800,000 frames. This is the most detailed infrared picture of our galaxy to date.]] .]] The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, itself being part of the Virgo Supercluster. Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a peculiar streamer of neutral hydrogen gas connecting these two small galaxies. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way. Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies, which are dynamically bound to the Milky Way, as well as some that have already been cannibalized by the Milky Way, such as Omega Centauri. Observations through the zone of avoidance are frequently detecting new distant and nearby galaxies. Some galaxies consisting mostly of gas and dust may also have evaded detection so far. In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they circle the Galaxy, causing vibrations at certain frequencies when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, by taking into account dark matter, the movement of these two galaxies creates a wake that influences the larger Milky Way. Taking dark matter into account results in an approximately twenty-fold increase in mass for the Galaxy. This calculation is according to a computer model made by Martin Weinberg of the University of Massachusetts, Amherst. In this model, the dark matter is spreading out from the galactic disc with the known gas layer. As a result, the model predicts that the gravitational effect of the Magellanic Clouds is amplified as they pass through the Galaxy. Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, individual stars within the galaxies would not collide, but instead the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years. Velocity for the Milky Way. Vertical axis is speed of rotation about the galactic center. Horizontal axis is distance from the galactic center in kpcs. The sun is marked with a yellow ball. The observed curve of speed of rotation is blue. The predicted curve based upon stellar mass and gas in the Milky Way is red. Scatter in observations roughly indicated by gray bars. The difference is due to dark matter or perhaps a modification of the law of gravity. ]] In the general sense, the absolute velocity of any object through space is not a meaningful question according to Einstein's special theory of relativity, which declares that there is no "preferred" inertial frame of reference in space with which to compare the Galaxy's motion. (Motion must always be specified with respect to another object.) Astronomers believe the Milky Way is moving at approximately 630 km per second relative to the local co-moving frame of reference that moves with the Hubble flow. If the Galaxy is moving at 600 km/s, Earth travels 51.84 million km per day, or more than 18.9 billion km per year, about 4.5 times its closest distance from Pluto. The Milky Way is thought to be moving in the direction of the Great Attractor. The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda galaxy) is part of a supercluster called the Local Supercluster, centered near the Virgo Cluster: although they are moving away from each other at 967 km/s as part of the Hubble flow, the velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster. Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s with respect to the photons of the CMB, toward 10.5 right ascension, -24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction. The galaxy rotates about its center according to its galaxy rotation curve as shown in the figure. The discrepancy between the observed curve (relatively flat) and the curve based upon the known mass of the stars and gas in the Milky Way (decaying curve) is attributed to dark matter. History Etymology and beliefs There are many creation myths around the world which explain the origin of the Milky Way and give it its name. The English phrase is a translation from Greek Γαλαξίας, Galaxias, which is derived from the word for milk (γάλα, gala). This is also the origin of the word galaxy. In Greek myth, the Milky Way was caused by milk spilt by Hera when suckling Heracles. The term Milky Way first appeared in English literature in a poem by Chaucer. }} In Sanskrit and several other Indo-Aryan languages, the Milky Way is called Akash Ganga (आकाशगंगा, Ganges of the heavens). The milky way is held to be sacred in the Hindu scriptures known as the Puranas, and the Ganges and the milky way are considered to be terrestrial-celestial analogs of each other. However, the term ''Kshira (क्षीर, milk) is also used as an alternative name for the milky way in Hindu texts. In a large area from Central Asia to Africa, the name for the Milky Way is related to the word for "straw". This may have originated in ancient Armenian mythology, (Յարդ զողի Ճանապարհ ''hard goghi chanaparh, or "Trail of the Straw Thief"), and been carried abroad by Arabs. – Scholar search }} In several Uralic, Turkic languages, Fenno-Ugric languages and in the Baltic languages the Milky Way is called the "Birds' Path" (Linnunrata in Finnish), since the route of the migratory birds appear to follow the Milky Way. (The Qi Xi legend celebrated in many Asian cultures references a seasonal bridge across the Milky Way formed by birds, usually magpies or crows.) The Chinese name "Silver River" (銀河) is used throughout East Asia, including Korea and Japan. An alternative name for the Milky Way in ancient China, especially in poems, is "Heavenly Han River"(天汉). In Japanese, "Silver River" (銀河 ginga) means galaxies in general and the Milky Way is called the "Silver River System" (銀河系 gingakei) or the "River of Heaven" (天の川 Amanokawa or Amanogawa). In Swedish, it is called Vintergatan, or "Winter Avenue", because the stars in the belt were used to predict time of the approaching winter. In some of the Iberian languages, the name refers to "Road of Saint James" Discovery .]] As Aristotle (384-322 BC) informs us in Meteorologica (DK 59 A80), the Greek philosophers Anaxagoras (ca. 500–428 BC) and Democritus (450–370 BC) proposed that the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions." The Arabian astronomer, Alhazen (965-1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it was very remote from the earth and did not belong to the atmosphere." The Persian astronomer, Abū Rayhān al-Bīrūnī (973-1048), proposed the Milky Way galaxy to be a collection of countless nebulous stars. Avempace (d. 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in the Earth's atmosphere. Ibn Qayyim Al-Jawziyya (1292-1350) proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars" and that that these stars are larger than planets. Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars. In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Galaxy with the Solar System close to the center. In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture. In 1917, Heber Curtis had observed the nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were actually independent galaxies. In 1920 the Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift. The matter was conclusively settled by Edwin Hubble in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence. See also *Galactic coordinate system *Dark matter halo *Smith's Cloud *Oort Constants *The Great Rift - A molecular dust cloud located between the solar system and the Sagittarius Arm of the Milky Way which appears to split the Milky Way into two lanes over a third of its length. References Further reading *Thorsten Dambeck in Sky and Telescope, "Gaia's Mission to the Milky Way", March 2008, p. 36–39. External links *The Milky Way Galaxy from An Atlas of the Universe *A 3D map of the Milky Way Galaxy *Milky Way – IRAS (infrared) survey wikisky.org *Milky Way – H-Alpha survey wikisky.org *Running Rings Around the Galaxy Spitzer Space Telescope News *The Milky Way Galaxy, SEDS Messier pages *MultiWavelength Milky Way, NASA site with images and VRML models *Milky Way Explorer, detailed images in infrared with radio, microwave and hydrogen-alpha as well *Face-on Milky Way maps, within about 10 thousand parsecs *The Milky Way at the Astro-Photography Site Of Mister T. Yoshida. *Widefield Image of the Summer Milky Way *Proposed Ring around the Milky Way *Milky Way spiral gets an extra arm, New Scientist.com *Possible New Milky Way Spiral Arm, Sky and Telescope.com *The Milky Way spiral arms and a possible climate connection *Galactic center mosaic via sun-orbiting Spitzer infrared telescope *Milky Way Plan Views, The University of Calgary Radio Astronomy Laboratory *Our Growing, Breathing Galaxy, Scientific American Magazine (January 2004 Issue) *Deriving The Shape Of The Galactic Stellar Disc, SkyNightly (March 17, 2006) *Digital Sky LLC, Digital Sky's Milky Way Panorama and other images *A new view of the Milky Way galaxy obtained by the Diffuse Infrared Background Experiment (DIRBE) on NASA's Cosmic Background Explorer satellite (COBE). *Image of Milky Way galaxy arms, Chandra X-ray Observatory Center *The 1920 Shapley – Curtis Debate on the size of the Milky Way *Milky Way Voyage – India's First & Largest Star Party *Astronomy Picture of the Day: **Composite image of the Milky Way **Milky Way Illustrated **Barred Spiral Milky Way (Illustrated) **Radioactive Clouds in the Milky Way **Milky Way Molecule Map **The Milky Way's Gamma-Ray Halo *Moving Milkyway seen from Teneriffe without any lightpollution *Multi-Gigapixel Infrared Milky Way A zoomable, annotated version of the Spitzer Space Telescope GLIMPSE survey. *Animated tour of the Milky Way, University of Glamorgan Category:Milky Way Galaxy Category:Milky Way Subgroup Category:Local Group Category:Galactic astronomy Category:Barred spiral galaxies Category:Spiral galaxies af:Melkweg ar:درب التبانة an:Carrera de San Chaime ast:Camín de Santiago az:Süd Yolu zh-min-nan:Gîn-hô-hē be-x-old:Млечны Шлях bs:Mliječni put bg:Млечен път ca:Via Làctia cv:Хуркайăк çулĕ cs:Galaxie Mléčná dráha cy:Llwybr Llaethog da:Mælkevejen de:Milchstraße et:Linnutee el:Γαλαξίας es:Vía Láctea eo:Lakta vojo eu:Esne Bidea fa:کهکشان راه شیری fr:Voie lactée ga:Bealach na Bó Finne gv:Raad Mooar Ree Gorree gl:Vía Láctea ko:우리 은하 hr:Mliječni put io:Lakto-voyo id:Bima Sakti os:Æрфæны фæд is:Vetrarbrautin it:Via Lattea he:שביל החלב jv:Bima Sakti kn:ಕ್ಷೀರಪಥ pam:Milky Way ka:ირმის ნახტომი la:Via lactea lv:Piena Ceļš lb:Mëllechstrooss lt:Paukščių Takas li:Mèlkweeg hu:Tejútrendszer mk:Млечен Пат ml:ആകാശഗംഗ mr:आकाशगंगा ms:Bima Sakti nl:Melkweg (sterrenstelsel) ne:आकाशगङ्गा ja:銀河系 no:Melkeveien nn:Mjølkevegen nrm:C'mîns d'Saint Jacques oc:Via Lactèa pl:Droga Mleczna pt:Via Láctea ro:Calea Lactee qu:Qullqaquyllur ru:Млечный Путь scn:Jolu di San Jàbbucu simple:Milky Way sd:کيرائين واٽ ڪهڪشان sk:Mliečna dráha sl:Rimska cesta (galaksija) sr:Млечни пут sh:Mliječna staza su:Bima Sakti fi:Linnunrata sv:Vintergatan tl:Ariwanas ta:பால் வழி te:పాలపుంత th:ทางช้างเผือก tr:Samanyolu tk:Akmaýanyň Ýoly uk:Чумацький Шлях ug:سامان يولى سىستىمېسى vi:Ngân Hà wa:Voye Sint-Djåke war:Gatasnon nga agianan zh-yue:銀河 bat-smg:Paukštiu Kel's zh:银河系 animation for details].]] The Orion Arm is a minor spiral arm of the Milky Way galaxy some 3,500 light years across and approximately 10,000 light years in length.Harold Spencer Jones, T. H. Huxley, Proceedings of the Royal Institution of Great Britain, Royal Institution of Great Britain, v. 38-39 The Solar System and Earth are within the Orion Arm. It is also referred to as the Local Arm, the Local Spur or the Orion Spur. The Orion Arm is named for its proximity to the stars in the Orion constellation. It is located between the Sagittarius Arm and the Perseus Arm, two of the four major arms of the Milky Way. Within the Orion Arm, the solar system and Earth are located close to the inner rim in the Local Bubble, about half-way along the Arm's length, approximately 8,000 parsecs (26,000 light-years) from the galactic center. Messier objects The Orion arm contains a number of Messier objects: *The Butterfly Cluster (M6) *The Ptolemy Cluster (M7) *Open Cluster M23 *Open Cluster M25 *The Dumbbell Nebula (M27) *Open Cluster M29 *Open Cluster M34 *Open Cluster M35 *Open Cluster M39 *Winnecke 4 (M40) *Open Cluster M41 *The Orion Nebula (M42) *The De Mairan's Nebula *The Beehive Cluster (M44) *The Pleiades (M45) *Open Cluster M46 *Open Cluster M47 *Open Cluster M48 *Open Cluster M50 *The Ring Nebula (M57) *Open Cluster M67 *M73 *The Little Dumbbell Nebula (M76) *Diffuse Nebula M78 *Open Cluster M93 *The Owl Nebula (M97) Interactive maps Image:Orion Arm.JPG|frame|center|Orion and neighboring arms (clickable map) rect 126 149 188 182 Rosette Nebula rect 285 116 327 145 Crab Nebula rect 243 245 284 274 Orion Nebula rect 299 288 345 312 Trifid Nebula rect 343 304 384 333 Lagoon Nebula rect 393 322 434 353 Omega Nebula rect 445 322 494 353 Eagle Nebula rect 424 244 483 280 North America Nebula rect 293 248 319 266 Rigel rect 225 179 299 246 The Three Kings rect 331 211 368 234 Polaris rect 318 236 353 259 Sun poly 302 176 303 241 315 242 361 177 Betelgeuse rect 419 222 458 245 Deneb poly 0 123 508 118 637 160 637 217 470 163 0 178 Perseus arm poly 2 202 460 201 633 261 637 326 408 260 1 258 Orion arm poly 1 284 397 293 633 360 637 477 541 475 357 413 0 400 Sagittarius Arm desc bottom-left Image:Nearest_Nebulae_and_Star_clusters.gif|thumb|800px|center|The nearest nebulae and star clusters (clickable map) rect 396 142 447 173 Rosette Nebula rect 376 230 426 258 Seagull Nebula rect 463 264 501 292 Cone Nebula rect 528 284 576 322 California Nebula rect 695 117 741 149 Heart Nebula rect 461 301 494 339 Orion Nebula rect 691 154 739 182 Soul Nebula rect 568 371 625 405 North America Nebula rect 643 366 687 402 Cocoon Nebula rect 688 392 761 429 Gamma Cygni Nebula rect 594 404 625 444 Veil Nebula rect 513 541 550 578 Trifid Nebula poly 676 435 690 435 692 427 723 429 726 461 677 461 Crescent Nebula rect 489 597 543 630 Lagoon Nebula rect 555 592 595 626 Omega Nebula rect 574 646 614 689 Eagle Nebula rect 444 633 500 678 Cat's Paw Nebula rect 90 502 161 529 Eta Carinae Nebula rect 442 37 491 69 Crab Nebula rect 517 158 547 175 Messier 37 rect 527 172 559 190 Messier 36 rect 533 191 563 208 Messier 38 rect 408 257 434 280 Messier 50 rect 327 232 357 257 Messier 46 rect 422 285 454 302 Messier 67 rect 553 321 582 338 Messier 34 rect 433 305 461 321 Messier 48 rect 409 314 435 330 Messier 41 rect 425 328 456 345 Messier 47 rect 474 343 500 365 Messier 44 rect 502 345 528 368 Messier 45 rect 542 378 568 399 Messier 37 rect 714 285 748 308 Messier 52 rect 352 285 379 308 Messier 93 rect 489 421 513 444 Messier 7 rect 495 452 518 473 Messier 6 rect 522 456 549 476 Messier 25 rect 512 478 539 500 Messier 23 rect 531 575 555 593 Messier 21 rect 556 564 589 580 Messier 18 rect 605 598 640 617 Messier 26 rect 630 618 654 639 Messier 11 rect 484 234 510 256 Messier 35 rect 287 248 316 276 NGC 2362 rect 370 359 411 381 IC 2395 rect 359 413 390 449 NGC 3114 rect 407 396 444 432 NGC 3532 rect 594 356 644 372 IC 1396 rect 458 392 502 406 IC 2602 rect 407 494 443 521 NGC 6087 rect 437 464 472 497 NGC 6025 rect 262 478 300 506 NGC 3766 rect 513 427 554 451 NGC 4665 rect 181 445 226 472 IC 2581 rect 212 506 257 526 IC 2944 rect 213 565 246 598 NGC 4755 rect 128 463 154 494 NGC 3293 rect 362 591 392 628 NGC 6067 rect 404 548 437 587 NGC 6193 rect 425 595 453 630 NGC 6231 rect 461 556 498 585 NGC 6383 rect 58 506 91 521 Tr 14 rect 77 520 108 536 Tr 16 rect 797 130 831 153 Messier 103 rect 665 408 691 434 Messier 29 rect 746 139 782 157 hPer rect 763 117 804 132 chi Per rect 152 485 194 500 Col 228 rect 456 377 492 393 o Vel poly 0 0 496 0 841 130 976 221 972 421 633 224 277 129 5 111 Perseus Arm poly 2 230 444 239 688 340 970 495 971 639 559 452 317 387 1 380 Orion Arm poly 2 481 423 492 694 606 922 757 470 761 234 669 1 668 Sagittarius Arm rect 879 666 965 684 Star Cluster rect 878 684 944 699 Nebula desc bottom-left See also * Gould Belt * Local Bubble * Loop I Bubble * List of Messier objects * List of nearest stars References External links *Messier Objects in the Milky Way (SEDS) *A 3D map of the Milky Way Galaxy be-x-old:Рукаў Арыёна ca:Braç d'Orió de:Orionarm es:Brazo de Orión fa:بازوی شکارچی fr:Bras d'Orion gv:Laue yn Çhelgeyr Vooar ko:오리온자리 팔 it:Braccio di Orione he:זרוע אוריון ms:Lengan Orion nl:Orionarm ja:オリオン腕 no:Orion-armen pl:Ramię Oriona pt:Braço de Órion ru:Рукав Ориона sk:Rameno Orióna sv:Orionarmen tr:Orion kolu zh:獵戶臂 Category:Orion arm Category:Milky Way Galaxy